Dispensing opticians often need to measure the optical power of a lens to duplicate prescriptions or distinguish between lenses. Opticians can calculate the optical power of a lens by measuring the curvature of the surfaces of the lens. Lenses constructed from different material have a different refractive index, and thus a lens with the same curvature but constructed from a different material will have a different optical power.
In order to measure the curvature of the lens and the lens power, a device that is referred to as a lens clock, lens gauge or clock gauge is used. Traditional lens clocks are mechanical devices that have a graduated dial and a needle to indicate the lens power of the device in diopters. The graduated dial of a typical lens clock is calibrated to display the optical power of crown glass which has a refractive index of 1.523. Nowadays few people actually wear lenses constructed from crown glass as a number of new materials are both lighter and less expensive. If an optician is measuring a lens made from a material other than crown glass, the optician must convert the diopter reading from the lens clock by applying a corrective factor. Applying this corrective factor can be a time consuming and possibly error prone task for the optician.
The construction of a traditional lens clock includes two fixed outer probes with a center probe in between that moves up and down against spring tension in a line perpendicular to the line between the two fixed pins. Failure to orient the probes perpendicular to the lens surface can affect the accuracy of the lens clock measurement. The center probe is integrated with a rack that rotates a pinion gear to translate the up and down motion of the plunger to the rotary motion of the needle that indicates the optical power on the graduated dial. The dial is typically graduated in quarter diopter increments that limits the precision and accuracy of the lens clock and can also be a source of observational errors. Mechanical variation and orientation of the pinion gear or rack due to temperature or manufacturing variance can also lead to measurement error that is compounded when making larger measurements that require more rotations of the pinion gear.
Prior to measuring the power of a lens using a traditional lens clock, the lens clock must be calibrated to read zero when the probes are placed on a flat surface. Typically, this requires the optician to manually adjust the center pin of the lens clock, usually by rotating the center pin, and then testing the adjustment by placing the lens clock on a flat surface. This process is repeated until the optician is satisfied that the lens clock is calibrated. This calibration process can be time consuming and another source of error affecting the measurements from the lens clock. Calibration also typically requires an additional tool to rotate the center pin.